The use of semiconductor components to limit voltages is known. Zener diodes (Z diodes), in particular, are used for this purpose. If Zener diodes are operated in the blocking or reverse direction, they display a pronounced breakdown behavior at comparatively low breakdown voltages. The value of the breakdown voltage of a diode depends substantially on the doping concentration of the semiconductor material. In highly doped diodes, a very narrow barrier layer forms, so that high electrical field strengths above the p-n transition are present upon application of even small reverse voltages.
If the field strength exceeds a value of approximately 106 V/cm, valence electrons in the vicinity of the almost charge-carrier-free p-n transition can be pulled out of their bonds. In the band model, this effect is represented as tunneling through the forbidden band. At low voltages below the breakdown voltage (also called the Zener voltage), only the (usually negligibly low) reverse current flows. When the Zener voltage is reached, the current rises sharply due to charge carrier emission, thus preventing any further increase in voltage. At breakdown voltages below 4.5 V, the result is a “pure Zener” breakdown. At higher breakdown voltages there is another competing breakdown effect, namely the so-called avalanche breakdown. This predominates at voltages above 7 V, and results substantially from avalanching impact ionizations in the semiconductor. Because of its defined and reversible breakdown, a Zener diode is suitable as a voltage limiter. If two Zener diodes are connected together in anti-serial fashion, i.e. in series but with opposite polarity, symmetrical breakdown behavior will be obtained.
A circuit of this kind is illustrated in FIG. 6, which depicts a first Zener diode 110 and a second Zener diode 112 connected anti-serially. Arrangements of this kind are used for voltage limiting in order to limit both polarities of a voltage applied to contacts 114, 116.
FIG. 7 shows the corresponding current/voltage characteristic of the circuit depicted in FIG. 6. In the diagram of FIG. 7, the current flowing through Zener diodes 110, 112 is plotted against the voltage applied to contacts 114, 116. Ignoring path resistances and the rise in breakdown voltage resulting from self-heating, the breakdown voltage of the arrangement is UZ1+UF, where UZ1 denotes the breakdown voltage of one of the Zener diodes (which in the present case are assumed to be identical) and UF is the voltage drop of a diode in the forward direction. If a voltage limiting circuit of this kind is to be designed for higher limit voltages, however, the positive breakdown voltage temperature response, as seen in FIG. 7, occurs. In FIG. 7, a solid line shows a characteristic at room temperature (RT) and a dashed line shows a characteristic at much higher temperature (HT). The positive temperature response seen here results principally from the fact that in diodes designed for higher breakdown voltages, avalanche breakdown is predominant.
The temperature dependence of the characteristic shown in FIG. 7 is undesirable. The voltage limiting circuit of FIG. 6 additionally has the disadvantage that two separate components are needed to implement it, entailing additional circuit complexity.